If your best friend is flirting with your boyfriend, then personally pull her aside from all of others and talk to her. If they don't listen to you, say you know it really bugs me when you talk to my boyfriend and maybe they will understand. Bring up an example and say you know you would not probably like it if I were talking to your boyfriend. Just remember to keep your cool and stay mature about it. The best thing you can do in this kind of situation is stay calm. If you get all worked up about it, she won't stop. So keep your cool.

Well it sounds like she isn't a really good friend. There are three things that you can try to solve this. Try to ignore what she says, eventually she will stop saying these things. Or simply tell her that if she keeps acting like this, you can't be friends anymore. Lastly you can tell her that you don't care what she says about you or him. If you ignore her and keep your cool, be mature about everything. If you don't show any sign of it annoying you, then she will back off.